Canvas is nice enough to paint on (I prefer it to painting on board), but I like to switch things up now and then. Jack's portrait is painted on monk's cloth, a material that is almost like a basket. Its flat "ribbons" of cotton are loosely woven together--the detail image may give you a better idea of what it looks like.
I have also painted a portrait on burlap, and I find myself working on bird's eye quite often. In fact, I like the texture of this flimsy cotton so well that I painted all of Swollen on it.

To learn more about the effects of painting on various materials, please take a look at my blog.